<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .zi{
            width: 100px;
            height: 100px;
            border: 1px solid red;
            position: relative;
            overflow: hidden;
        }
        .meng{
        
            width: 100px;
            height: 100px;
            border: 1px solid red;
            background: black;
            position: absolute;
            top: 100px;
            transition: all 2s;
        }
        .zi:hover .meng{
            top: 0px;
        }
    </style>
</head>
<body>
    <div class="zi">
        <div class="meng"></div>
    </div>
    
</body>
</html>